Condensates and waste water in which relatively large amounts of environmentally harmful impurities are dissolved result from the degassing and gasification of coal. Such aqueous solutions cannot be introduced directly into conventional purification plants, rivers, or other water receivers. Rather, it is necessary for these harmful substances to be removed beforehand, in which case the strict limitations of the environmental legislation regarding maximum harmful substance concentrations must be observed.
The impurities present in solution are organic and inorganic in nature. The organic substances include phenols and other oxygen-containing compounds, such as ketones, aldehydes, carboxylic acids, and nitrogen-containing compounds, such as pyridine derivatives and sulfur compounds. Undesirable inorganic substances which are found in solution are mainly hydrogen sulfide, hydrogen cyanide, and ammonia, which are present as such or in the form of compounds.
It is known that organic substances dissolved in waste water can be extracted with solvents which are immiscible, or miscible to only a small extent, with water. The inorganic impurities are preferably separated off with steam or inert gases, a procedure for which various embodiments have been developed.
Thus, according to DE-B 36 284, hydrogen sulfide and hydrogen cyanide are removed continuously from acid coking plant waste water or similar waste water in a two-stage process by blowing with substantially inert gases, such as air.
DE 22 29 213 C2 describes a process for treating waste water from gasification of coal or degassing of coal which is based on the removal of carbon dioxide, hydrogen sulfide, and ammonia from the solution by means of steam. This publication does not deal with the treatment of waste water containing hydrogen cyanide.
The removal of acid gases, in particular carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen cyanide, and their mixtures, as well as free and bonded ammonia, from dilute aqueous solutions is the subject matter of DE 26 52 524 C2. The process comprises a two-stage continuous distillation with stripping steam passed in the counter-current direction.
The processes described lead to significant reductions in the concentrations of harmful substances in aqueous solutions containing these impurities, so that the maximum values permitted by law can be satisfied. However, it has been found that the low cyanide levels initially achieved gradually rise again in contrast to those of the other impurities.
Methods other than distillation and its variants have also been used specifically for the removal of cyanides or hydrogen cyanide from aqueous solutions; chemical processes have proven to be particularly suitable for this purpose. The solutions are treated with hypochlorite, with peroxy compounds, or with ozone; or the cyanides are hydrolyzed at temperatures of 180.degree. C. to more than 200.degree. C. under pressure. One of the oldest chemical processes is based on the reaction of cyanide with iron (II) salts in an alkaline medium. However, even these processes do not always produce the desired result, or they have only limited use for economic reasons.
It is an object of the invention to provide a procedure which eliminates the deficiencies described and enables hydrogen sulfide, hydrogen cyanide, and ammonia to be separated effectively from their aqueous solutions.